1. Field of the Invention
The present invention relates to a developing device used in image forming apparatus such as electrophotographic printers and copiers.
2. Description of Related Art
An example of a two-component system developing device used in a conventional electrophotographic image forming apparatus will be described referring to FIGS. 7 and 8. FIG. 7 is a schematic perspective view of the conventional developing device, and FIG. 8 is a sectional view taken along a line VIII—VIII and seen in an arrow direction in FIG. 7.
A developer carrier 1 is disposed so as to be in the vicinity of or in contact with a latent image carrier 6, which is not shown in FIG. 7. The developer carrier 1 includes a fixed magnet 1b having a plurality of magnetic poles therein and a developing roller 1a that is provided around the fixed magnet 1b and rotates in a direction of an arrow A. A two-component developer 2 containing a toner and a magnetic carrier is supplied to the perimeter surface of the developing roller 1a. The developer that is regulated to have a constant thickness by a regulating blade 5 and adheres to the surface of the developing roller 1a is carried to a developing region 30 by the rotation of the developing roller 1a. 
In the developing region 30, a developing electric field generated by a bias voltage applied to the developing roller 1a allows a charged toner to move to the latent image carrier 6, so that a static latent image formed on the latent image carrier 6 is developed.
An electric field generated by the developed toner cancels out the developing electric field. Accordingly, when the amount of the toner adhering to the latent image carrier 6 increases so that the electric field thereof and the developing electric field balance out, the development ends at this point. Thus, since a toner with a high charge level generates a large electric field, the toner amount necessary for development is small, leading to a low image density. Conversely, since a toner with a low charge level generates a small electric field, the toner amount necessary for development is large, leading to a high image density.
The charge level of the toner in the developer increases in keeping with stirring time and reaches a certain saturation amount at a certain stirring time. A developer with a low toner concentration has a high charge level in saturation and thus achieves a low image density, whereas a developer with a high toner concentration has a low charge level in saturation and thus the toner easily stains the image or is scattered across the image. Accordingly, there has been a problem that insufficient or non-uniform stirring of the developer in the developing device causes concentration unevenness and charging unevenness of the toner, leading to unevenness and variation in the image density.
In order to solve this problem, a developing device is known in which a pair of stirring screws 3 and 4, between which a partition plate 8 is interposed, are provided as members for stirring the developer 2 as shown in FIGS. 7 and 8. The rotations of the stirring screws 3 and 4 respectively stir and carry the developer 2 along rotating-shaft directions indicated by arrows B and C, which are opposite to each other (see JP 10(1998)-239970 A, for example). At both ends of the rotating shafts, the partition plate 8 is cut out so as to form transfer regions 14 for the developer 2, and the developer 2 moves from one stirring screw to the other stirring screw, so that a path through which the developer 2 circulates is formed along both of the stirring screws 3 and 4. Since the developer 2 is stirred and charged while being carried through the circulating path, the developer 2 that is supplied from the stirring screw 3 to the developing roller 1a is supposed to achieve a uniform image with a constant density. Further, replacement toner in an amount that is as much as the toner consumed for development is newly supplied to the circulating path and stirred with the developer remaining in the circulating path. Thus, the developer maintains a constant toner concentration and a constant charge level, and a stable image is supposed to be obtained.
However, in an image forming apparatus that has been made smaller and thinner as a whole and a color printer including plural developing devices, a space that a developing device can occupy has become limited. Accordingly, the size of the developing device and the amount of the developer that can be put therein have been reduced. Consequently, there has been a problem that the developer is deteriorated in a short period of time, making it necessary to replace the developer in the developing device or the developing device itself frequently.
In order to extend the lifetime of the developing device, a technology called “trickle” is known, in which a certain proportion of the developer always is discharged from the developing device and a corresponding amount of the developer is newly supplied to the developing device. However, this technique is inefficient because the developer that is still usable also is discharged and thrown away, and the necessary amount of the developer increases, leading to a cost increase.